THE EAR OF MAN. 



201 



into its anterior and posterior halves, which, owing to 

 the distortion produced by the enormous growth of the 



Fig. 6. — The left internal ear of a 

 human embryo, 22 mm. in length, 

 seen from without and below. Figure 

 after W. His., Jr. The figure is from 

 a model constructed from serial sec- 

 tions, and represents the ear much 

 magnified. 



a Anterior canal. 

 am Ampulla. 



am' Amae. The middle reference line is 

 superfluous. 



Cochlea. 



Ductus endolymphaticus. 



External canal. 



Sacculus in the restricted sense; really 

 only the recessus sacculi. 



Utriculus. 



c 

 d 

 h 

 s 



u 



cochlea, now appear as superior and inferior portions of 

 the canal system of the ear. 



The development of the special sense-organs of the 

 lateral line in A^nm or the ganoid Dog-fish, as made 

 known by Allis, gives us the key to the solution of the 

 problem of the homologies of the parts of the internal 

 ear. In this fish the inclosure of the canals and the 

 formation of the pores and tubes, while it is undoubtedly 

 the primitive process, is essentially a simple and regular 

 process, and when it is fully carried out t/ie canals arise 

 in separate sections^ each of which contains a single sense- 



organ. 



In the young Dog-fish, in which the canals have not 

 begun their development, the sense-organs lie below the 

 surface and may be traced as more or less continuous 

 whitish lines. "These lines mark general and exten- 

 sive surface depressions." From the bottom of these 



